.\"
.\"	aegis - project change supervisor
.\"	Copyright (C) 1993, 1994 Peter Miller.
.\"	All rights reserved.
.\"
.\"	This program is free software; you can redistribute it and/or modify
.\"	it under the terms of the GNU General Public License as published by
.\"	the Free Software Foundation; either version 2 of the License, or
.\"	(at your option) any later version.
.\"
.\"	This program is distributed in the hope that it will be useful,
.\"	but WITHOUT ANY WARRANTY; without even the implied warranty of
.\"	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
.\"	GNU General Public License for more details.
.\"
.\"	You should have received a copy of the GNU General Public License
.\"	along with this program; if not, write to the Free Software
.\"	Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111, USA.
.\"
.\" MANIFEST: User Guide, The Dependency Maintenance Tool, Requirements
.nh 2 "Required Features"
.LP
When selecting a Dependency Maintenance Tool
it is important to keep in mind that, ideally, it must be able to
cope with a hierarchy of parallel source directory trees.
.LP
The heart of any DMT is an \fIinference engine\fP.
This inference engine accepts a \fIgoal\fP of what you want it to construct
and a set of \fIrules\fP for how to construct things,
and attempts to construct what you asked for given the rules you specified.
This is exactly a description of an expert system,
and the DMT needs to be an expert system for constructing files.
.LP
This perspective on what the aegis program needs from a DMT
reveals that the old-faithful \fImake\fP(1) distributed
with so many flavours of 
.UX
really isn't good enough,
and that something like PROLOG is probably ideal.
.nh 3 "Search Lists"
.LP
For the union of all files in a project and all files in a change
(remembering that a change only copies those files it is modifying,
plus it may add or remove files)
for all files you must be able to say to the dependency maintenance
tool,
.QP
"If and only if the file is up-to-date in the baseline,
use the baseline copy of the file,
otherwise construct the file in the development directory".
.LP
The presence of a source file in the change makes the copy
in the baseline out-of-date.
.LP
Most DMTs with this capability implement it by using
some sort of search path,
allowing a hierarchy of directories to be scanned with little or
no modification to the rules.
.LP
If your DMT of choice does not provide this functionality,
the
.I create_symlinks_before_build
field of the project
.I config
file may be set to
\fItrue\fP,
which tells aegis to maintain symbolic links in the development directory
for all files in the baseline which are not present in
the development directory.
(See
\fIaepconf\fP(5)
and
\fIaeb\fP(1)
for more information.)
This incurs a certain amount of overhead when aegis maintains these links,
but a similar amount of work is done within DMTs which have search path
functionality.
.nh 3 "Dynamic Include File Dependencies"
.LP
Include file dependencies are very important,
because a change may alter an include file,
and all of the sources in the baseline which use that include file
must be recompiled.
.LP
Consider the example given earlier:
the include file describing the interface definition of a function is
copied into a change and edited,
and so is the source file defining the function.
It is essential that all source files in the baseline which
include that file re recompiled,
which will usually result in suitable diagnostic errors if any of the
clients of the altered function have yet to be included in the change.
.LP
There are two ways of handling include file dependencies:
.LP
\(bu They can be kept in a file,
and the file can be maintained by suitable programs
(maintaining it manually never works, that's just human nature).
.LP
\(bu They can be determined by the DMT when it is scanning the rules
to determine what needs updating.
.nh 4 "Static File"
.LP
Keeping include dependencies in a file has a number of advantages:
.LP
\(bu Most existing DMTs have the ability to include other rules files,
so that when performing a development build
from a baseline rules file,
it could include a
dependencies file in the development directory.
.LP
\(bu Reading a file is much faster than scanning all of the source files.
.LP
Keeping include dependencies in a file has a number of disadvantages:
.LP
\(bu The file is independent of the DMT,
it is either generated before the DMT is invoked,
in which case it may do more work than is necessary,
or it may be invoked after the DMT (or after the DMT has scanned its rules),
in which case it may well be out-of-date when the DMT needs it.
.LP
For example,
the use of
.I "gcc -M"
produces "dot d" files,
which may be merged to 
construct such an includable dependency file.
This happens after the DMT has read and applied the rules,
but possibly before the DMT has finished executing.\**
.FS
See the
.I "Using Make"
section for how GNU Make may be used.
It effectively combines both methods:
keeping \fI.d\fP files and dynamically updating them.
Because it combines both methods,
it has some of the advantages and disadvantages of both.
.FE
.LP
\(bu Many tools which can generate this information,
such as the
.I "gcc -M"
option,
are triggered by source files,
and are unable to manage a case where it is an include file which is changing,
to include a different set of other include files.
In this case,
the inaccurate dependencies file may contain references to 
the old set of nested include files,
some of which may no longer exist,
This causes
the DMT to incorrectly generate an error stating that the
old include file is missing,
when it is actually no longer required.
.LP
If a DMT can only support this kind of include file dependencies,
it is not suitable for use with aegis.
.nh 4 "Dynamic"
.LP
In order for a DMT to be suitable for use with aegis,
it is essential that rules for the DMT may be specified in such a way that
include file dependencies are determined "on the fly"
when the DMT is determining if a given rule is applicable,
and before the rule is applied.
.LP
This method suffers from the problem being rather slow;
but this is amenable to some caching and the losses of performance
are not as bad as could be imagined.
.LP
This method has the advantage of correctness in all cases,
where a static file may at times be out-of-date.